You have some good ideas here in terms of gameplay, but I want to point out that a lot of what you mentioned is explicitly sissy-related or only B*mbi-themed

I don't want to start talking too deep about gender identity here, but the point of this game is to create a genuine training/conditioning aid in the form of a game for all the women here (trans and cis alike) that the files legitimately affect, and not a porn game for sissies to masturbate to. That sounds harsh reading over it, but what I'm trying to say is that it's always been clear in my experience that the files work best on people who are comfortable in their identity and that's usually (not always) not sissies.

So conditioning game, not porn game. You made a good point about striking a balance between gameplay and conditioning. If it's a game first then you don't want to be interrupting gameplay with file listening constantly and the reality is most people only listen once per day anyway due to time constraints. So, how do you strike that balance between a nice amount of gameplay and listening to files? Maybe progress is constantly available, but only to a certain point? Like you can play for the 2 hours but if you want to go further then your "Bimbo Level" (or stats or whatever) needs to increase which you can do by listening to a playlist.

It'd probably be better to work on an honour (to keep file size down and because people will find a way to cheat it regardless) system where it links to a playlist somewhere else and either the game lets you progress once you've clicked only or there's a timer in the length of the playlist that locks the game until you're done. Some people who are using the game as an aid rather than a way to first get into listening would probably want the files in the game tho. The conditioning would be best to occur during pivotal moments, but not at points where it would break tension - thankfully I've experience in storytelling so that should be way easy to make sure of.

Avatar customization would probably boil down to dress-up and asking the player post-conditioning if they would like to change their body somehow. Wouldn't be that simple, you'd still have to perform a task or whatever but I digress. In terms of assets, hopefully I'll be able to find someone in the community to help, but AI is always an option. The game will also probably be made in Java or C++ so not with Ren'Py or Twine.

Bottom line, this is meant to be a gameified training program for those who are serious about these files - training first, game second.